The general advice is that you should not call GC.Collect from your code, but what are the exceptions to this rule?
I can only think of a few very specific cases where it may make sense to force a garbage collection.
One example that springs to mind is a service, that wakes up at intervals, performs some task, and then sleeps for a long time. In this case, it may be a good idea to force a collect to prevent the soon-to-be-idle process from holding on to more memory than needed.
Are there any other cases where it is acceptable to call GC.Collect?
If you have good reason to believe that a significant set of objects - particularly those you suspect to be in generations 1 and 2 - are now eligible for garbage collection, and that now would be an appropriate time to collect in terms of the small performance hit.
A good example of this is if you've just closed a large form. You know that all the UI controls can now be garbage collected, and a very short pause as the form is closed probably won't be noticeable to the user.
UPDATE 2.7.2018
As of .NET 4.5 - there is GCLatencyMode.LowLatency and GCLatencyMode.SustainedLowLatency. When entering and leaving either of these modes, it is recommended that you force a full GC with GC.Collect(2, GCCollectionMode.Forced).
As of .NET 4.6 - there is the GC.TryStartNoGCRegion method (used to set the read-only value GCLatencyMode.NoGCRegion). This can itself, perform a full blocking garbage collection in an attempt to free enough memory, but given we are disallowing GC for a period, I would argue it is also a good idea to perform full GC before and after.
Source: Microsoft engineer Ben Watson's: Writing High-Performance .NET Code, 2nd Ed. 2018.
See:
https://msdn.microsoft.com/en-us/library/system.runtime.gclatencymode(v=vs.110).aspx
https://msdn.microsoft.com/en-us/library/dn906204(v=vs.110).aspx
I use GC.Collect only when writing crude performance/profiler test rigs; i.e. I have two (or more) blocks of code to test - something like:
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestA(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
TestB(); // may allocate lots of transient objects
GC.Collect(GC.MaxGeneration, GCCollectionMode.Forced);
...
So that TestA() and TestB() run with as similar state as possible - i.e. TestB() doesn't get hammered just because TestA left it very close to the tipping point.
A classic example would be a simple console exe (a Main method sort-enough to be posted here for example), that shows the difference between looped string concatenation and StringBuilder.
If I need something precise, then this would be two completely independent tests - but often this is enough if we just want to minimize (or normalize) the GC during the tests to get a rough feel for the behaviour.
During production code? I have yet to use it ;-p
The best practise is to not force a garbage collection in most cases. (Every system I have worked on that had forced garbage collections, had underlining problems that if solved would have removed the need to forced the garbage collection, and speeded the system up greatly.)
There are a few cases when you know more about memory usage then the garbage collector does. This is unlikely to be true in a multi user application, or a service that is responding to more then one request at a time.
However in some batch type processing you do know more then the GC. E.g. consider an application that.
Is given a list of file names on the command line
Processes a single file then write the result out to a results file.
While processing the file, creates a lot of interlinked objects that can not be collected until the processing of the file have complete (e.g. a parse tree)
Does not keep match state between the files it has processed.
You may be able to make a case (after careful) testing that you should force a full garbage collection after you have process each file.
Another cases is a service that wakes up every few minutes to process some items, and does not keep any state while it’s asleep. Then forcing a full collection just before going to sleep may be worthwhile.
The only time I would consider forcing
a collection is when I know that a lot
of object had been created recently
and very few objects are currently
referenced.
I would rather have a garbage collection API when I could give it hints about this type of thing without having to force a GC my self.
See also "Rico Mariani's Performance Tidbits"
These days I consider same of the above cases would be better to use a short lived worker process to do each batch of work and let the OS do the resource recovery.
One case is when you are trying to unit test code that uses WeakReference.
In large 24/7 or 24/6 systems -- systems that react to messages, RPC requests or that poll a database or process continuously -- it is useful to have a way to identify memory leaks. For this, I tend to add a mechanism to the application to temporarily suspend any processing and then perform full garbage collection. This puts the system into a quiescent state where the memory remaining is either legitimately long lived memory (caches, configuration, &c.) or else is 'leaked' (objects that are not expected or desired to be rooted but actually are).
Having this mechanism makes it a lot easier to profile memory usage as the reports will not be clouded with noise from active processing.
To be sure you get all of the garbage, you need to perform two collections:
GC.Collect();
GC.WaitForPendingFinalizers();
GC.Collect();
As the first collection will cause any objects with finalizers to be finalized (but not actually garbage collect these objects). The second GC will garbage collect these finalized objects.
You can call GC.Collect() when you know something about the nature of the app the garbage collector doesn't.
As the author, it's often tempting to think this is likely or normal. However, the truth is the GC amounts to a pretty well-written and tested expert system, and it's rare you'll know something about the low level code paths it doesn't.
The best example I can think of where you might have some extra information is an app that cycles between idle periods and very busy periods. You want the best performance possible for the busy periods and therefore want to use the idle time to do some clean up.
However, most of the time the GC is smart enough to do this anyway.
One instance where it is almost necessary to call GC.Collect() is when automating Microsoft Office through Interop. COM objects for Office don't like to automatically release and can result in the instances of the Office product taking up very large amounts of memory. I'm not sure if this is an issue or by design. There's lots of posts about this topic around the internet so I won't go into too much detail.
When programming using Interop, every single COM object should be manually released, usually though the use of Marshal.ReleseComObject(). In addition, calling Garbage Collection manually can help "clean up" a bit. Calling the following code when you're done with Interop objects seems to help quite a bit:
GC.Collect()
GC.WaitForPendingFinalizers()
GC.Collect()
In my personal experience, using a combination of ReleaseComObject and manually calling garbage collection greatly reduces the memory usage of Office products, specifically Excel.
As a memory fragmentation solution.
I was getting out of memory exceptions while writing a lot of data into a memory stream (reading from a network stream). The data was written in 8K chunks. After reaching 128M there was exception even though there was a lot of memory available (but it was fragmented). Calling GC.Collect() solved the issue. I was able to handle over 1G after the fix.
Have a look at this article by Rico Mariani. He gives two rules when to call GC.Collect (rule 1 is: "Don't"):
When to call GC.Collect()
I was doing some performance testing on array and list:
private static int count = 100000000;
private static List<int> GetSomeNumbers_List_int()
{
var lstNumbers = new List<int>();
for(var i = 1; i <= count; i++)
{
lstNumbers.Add(i);
}
return lstNumbers;
}
private static int[] GetSomeNumbers_Array()
{
var lstNumbers = new int[count];
for (var i = 1; i <= count; i++)
{
lstNumbers[i-1] = i + 1;
}
return lstNumbers;
}
private static int[] GetSomeNumbers_Enumerable_Range()
{
return Enumerable.Range(1, count).ToArray();
}
static void performance_100_Million()
{
var sw = new Stopwatch();
sw.Start();
var numbers1 = GetSomeNumbers_List_int();
sw.Stop();
//numbers1 = null;
//GC.Collect();
Console.WriteLine(String.Format("\"List<int>\" took {0} milliseconds", sw.ElapsedMilliseconds));
sw.Reset();
sw.Start();
var numbers2 = GetSomeNumbers_Array();
sw.Stop();
//numbers2 = null;
//GC.Collect();
Console.WriteLine(String.Format("\"int[]\" took {0} milliseconds", sw.ElapsedMilliseconds));
sw.Reset();
sw.Start();
//getting System.OutOfMemoryException in GetSomeNumbers_Enumerable_Range method
var numbers3 = GetSomeNumbers_Enumerable_Range();
sw.Stop();
//numbers3 = null;
//GC.Collect();
Console.WriteLine(String.Format("\"int[]\" Enumerable.Range took {0} milliseconds", sw.ElapsedMilliseconds));
}
and I got OutOfMemoryException in GetSomeNumbers_Enumerable_Range method the only workaround is to deallocate the memory by:
numbers = null;
GC.Collect();
You should try to avoid using GC.Collect() since its very expensive. Here is an example:
public void ClearFrame(ulong timeStamp)
{
if (RecordSet.Count <= 0) return;
if (Limit == false)
{
var seconds = (timeStamp - RecordSet[0].TimeStamp)/1000;
if (seconds <= _preFramesTime) return;
Limit = true;
do
{
RecordSet.Remove(RecordSet[0]);
} while (((timeStamp - RecordSet[0].TimeStamp) / 1000) > _preFramesTime);
}
else
{
RecordSet.Remove(RecordSet[0]);
}
GC.Collect(); // AVOID
}
TEST RESULT: CPU USAGE 12%
When you change to this:
public void ClearFrame(ulong timeStamp)
{
if (RecordSet.Count <= 0) return;
if (Limit == false)
{
var seconds = (timeStamp - RecordSet[0].TimeStamp)/1000;
if (seconds <= _preFramesTime) return;
Limit = true;
do
{
RecordSet[0].Dispose(); // Bitmap destroyed!
RecordSet.Remove(RecordSet[0]);
} while (((timeStamp - RecordSet[0].TimeStamp) / 1000) > _preFramesTime);
}
else
{
RecordSet[0].Dispose(); // Bitmap destroyed!
RecordSet.Remove(RecordSet[0]);
}
//GC.Collect();
}
TEST RESULT: CPU USAGE 2-3%
In your example, I think that calling GC.Collect isn't the issue, but rather there is a design issue.
If you are going to wake up at intervals, (set times) then your program should be crafted for a single execution (perform the task once) and then terminate. Then, you set the program up as a scheduled task to run at the scheduled intervals.
This way, you don't have to concern yourself with calling GC.Collect, (which you should rarely if ever, have to do).
That being said, Rico Mariani has a great blog post on this subject, which can be found here:
http://blogs.msdn.com/ricom/archive/2004/11/29/271829.aspx
One useful place to call GC.Collect() is in a unit test when you want to verify that you are not creating a memory leak (e. g. if you are doing something with WeakReferences or ConditionalWeakTable, dynamically generated code, etc).
For example, I have a few tests like:
WeakReference w = CodeThatShouldNotMemoryLeak();
Assert.IsTrue(w.IsAlive);
GC.Collect();
GC.WaitForPendingFinalizers();
Assert.IsFalse(w.IsAlive);
It could be argued that using WeakReferences is a problem in and of itself, but it seems that if you are creating a system that relies on such behavior then calling GC.Collect() is a good way to verify such code.
There are some situations where it is better safe than sorry.
Here is one situation.
It is possible to author an unmanaged DLL in C# using IL rewrites (because there are situations where this is necessary).
Now suppose, for example, the DLL creates an array of bytes at the class level - because many of the exported functions need access to such. What happens when the DLL is unloaded? Is the garbage collector automatically called at that point? I don't know, but being an unmanaged DLL it is entirely possible the GC isn't called. And it would be a big problem if it wasn't called. When the DLL is unloaded so too would be the garbage collector - so who is going to be responsible for collecting any possible garbage and how would they do it? Better to employ C#'s garbage collector. Have a cleanup function (available to the DLL client) where the class level variables are set to null and the garbage collector called.
Better safe than sorry.
The short answer is: never!
using(var stream = new MemoryStream())
{
bitmap.Save(stream, ImageFormat.Png);
techObject.Last().Image = Image.FromStream(stream);
bitmap.Dispose();
// Without this code, I had an OutOfMemory exception.
GC.Collect();
GC.WaitForPendingFinalizers();
//
}
Another reason is when you have a SerialPort opened on a USB COM port, and then the USB device is unplugged. Because the SerialPort was opened, the resource holds a reference to the previously connected port in the system's registry. The system's registry will then contain stale data, so the list of available ports will be wrong. Therefore the port must be closed.
Calling SerialPort.Close() on the port calls Dispose() on the object, but it remains in memory until garbage collection actually runs, causing the registry to remain stale until the garbage collector decides to release the resource.
From https://stackoverflow.com/a/58810699/8685342:
try
{
if (port != null)
port.Close(); //this will throw an exception if the port was unplugged
}
catch (Exception ex) //of type 'System.IO.IOException'
{
System.GC.Collect();
System.GC.WaitForPendingFinalizers();
}
port = null;
If you are creating a lot of new System.Drawing.Bitmap objects, the Garbage Collector doesn't clear them. Eventually GDI+ will think you are running out of memory and will throw a "The parameter is not valid" exception. Calling GC.Collect() every so often (not too often!) seems to resolve this issue.
i am still pretty unsure about this.
I am working since 7 years on an Application Server. Our bigger installations take use of 24 GB Ram. Its hightly Multithreaded, and ALL calls for GC.Collect() ran into really terrible performance issues.
Many third party Components used GC.Collect() when they thought it was clever to do this right now.
So a simple bunch of Excel-Reports blocked the App Server for all threads several times a minute.
We had to refactor all the 3rd Party Components in order to remove the GC.Collect() calls, and all worked fine after doing this.
But i am running Servers on Win32 as well, and here i started to take heavy use of GC.Collect() after getting a OutOfMemoryException.
But i am also pretty unsure about this, because i often noticed, when i get a OOM on 32 Bit, and i retry to run the same Operation again, without calling GC.Collect(), it just worked fine.
One thing i wonder is the OOM Exception itself...
If i would have written the .Net Framework, and i can't alloc a memory block, i would use GC.Collect(), defrag memory (??), try again, and if i still cant find a free memory block, then i would throw the OOM-Exception.
Or at least make this behavior as configurable option, due the drawbacks of the performance issue with GC.Collect.
Now i have lots of code like this in my app to "solve" the problem:
public static TResult ExecuteOOMAware<T1, T2, TResult>(Func<T1,T2 ,TResult> func, T1 a1, T2 a2)
{
int oomCounter = 0;
int maxOOMRetries = 10;
do
{
try
{
return func(a1, a2);
}
catch (OutOfMemoryException)
{
oomCounter++;
if (maxOOMRetries > 10)
{
throw;
}
else
{
Log.Info("OutOfMemory-Exception caught, Trying to fix. Counter: " + oomCounter.ToString());
System.Threading.Thread.Sleep(TimeSpan.FromSeconds(oomCounter * 10));
GC.Collect();
}
}
} while (oomCounter < maxOOMRetries);
// never gets hitted.
return default(TResult);
}
(Note that the Thread.Sleep() behavior is a really App apecific behavior, because we are running a ORM Caching Service, and the service takes some time to release all the cached objects, if RAM exceeds some predefined values. so it waits a few seconds the first time, and has increased waiting time each occurence of OOM.)
one good reason for calling GC is on small ARM computers with little memory, like the Raspberry PI (running with mono).
If unallocated memory fragments use too much of the system RAM, then the Linux OS can get unstable.
I have an application where I have to call GC every second (!) to get rid of memory overflow problems.
Another good solution is to dispose objects when they are no longer needed. Unfortunately this is not so easy in many cases.
This isn't that relevant to the question, but for XSLT transforms in .NET (XSLCompiledTranform) then you might have no choice. Another candidate is the MSHTML control.
If you are using a version of .net less than 4.5, manual collection may be inevitable (especially if you are dealing with many 'large objects').
this link describes why:
https://blogs.msdn.microsoft.com/dotnet/2011/10/03/large-object-heap-improvements-in-net-4-5/
Since there are Small object heap(SOH) and Large object heap(LOH)
We can call GC.Collect() to clear de-reference object in SOP, and move lived object to next generation.
In .net4.5, we can also compact LOH by using largeobjectheapcompactionmode
I'm running a monte carlo simulation.
Work is partitioned among many different machines (typically around 150).
After each iteration, each worker sends its results to the server.
After getting results from all workers, server calculates an update and sends it back to all workers.
This cycle repeats for 100-1000 iterations.
Server cannot compute update until all workers send their results, so if 99 workers take 1 second to finish an iteration and 100th worker takes 10 seconds, then entire iteration takes 10 seconds.
The problem is that GC randomly kicks in on some workers on some iterations, so causing these workers to take more time and thus slowing the entire process down.
For example during iteration #1 worker #58 took 10 seconds, where other workers took 8 seconds. On iteration #2 different worker takes longer and so on.
This added overhead seems to be around 20-30%.
What I would like to do is to instruct GC not to do any collections while iteration is taking place.
Collect only every say 10 iterations (so that all workers synchronize their collections), or collect after sending results, and before getting an update from the server.
Here is a pseudocode for what I'm trying to do:
public void Algorithm()
{
for (var iteration = 0; iteration < 1000; iteration++)
{
PerformIteration(); //don't do any GC inside.
SendResults();
//Now there is a small time window to perform GC
//before results from the server arrive (thats usually sub 0.5sec window)
WaitForUpdate();
}
}
Setting: GCSettings.LatencyMode = GCLatencyMode.SustainedLowLatency helped quite a bit, but significant overhead still remains.
Each worker has 244gb of ram, considerably more than simulation requires.
Also, almost everything is cached, so no need to do Gen2 collections.
.NET 4.6 has a new GC feature called GC.TryStartNoGCRegion.
This tells the GC to attempt to run this block of code without doing any collections at all:
Attempts to disallow garbage collection during the execution of a
critical path if a specified amount of memory is available, and
controls whether the garbage collector does a full blocking garbage
collection if not enough memory is initially available.
When you invoke it, you instruct the GC how much memory you can allocate before it has to perform a GC. It must be less than or equal to the ephemeral segments size:
public void Algorithm()
{
for (var iteration = 0; iteration < 1000; iteration++)
{
// allow the GC to allocate 4kb
if (GC.TryStartNoGCRegion(4096, true))
{
try
{
PerformIteration();
SendResults();
}
finally
{
GC.EndNoGCRegion();
}
}
//Now there is a small time window to perform GC
//before results from the server arrive (thats usually sub 0.5sec window)
WaitForUpdate();
}
}
-You can use unmanaged code on C# with this (GC.AddMemoryPressure)
-or use managed GC blocks
GC.TryStartNoGCRegion(.)
{
// your critical code
} GC.EndNoGCRegion()
*don't forgot check Try-condition as on previous answer
-GCSettings.LatencyMode look at MS manual
-change GC configuration in *.config file (app or machine) with gcServer and gcConcurrent params
<configuration>
<runtime>
<gcServer enabled="true"/>
<!-- OR / AND -->
<gcConcurrent enabled="true|false"/>
</runtime>
</configuration>
I am trying to track down a memory leak in a larger C# program which spawns multiple threads. In the process, I have created a small side program which I am using to test some basic things, and I found some behavior that I really do not understand.
class Program
{
static void test()
{
}
static void Main(string[] args)
{
while (true)
{
Thread test_thread = new Thread(() => test());
test_thread.Start();
Thread.Sleep(20);
}
}
}
Running this program, I see that the memory usage of the program increases steadily without stopping. In just a few minutes the memory usage goes well over 100MB and keeps climbing. If I comment out the line test_thread.Start();, the memory used by the program maxes out at about a few megabytes, and levels out. I also tried forcing garbage collection at the end of the while loop using GC.Collect(), but it did not seem to do anything.
I thought that the thread would be dereferenced as soon as the function is finished executing allowing the GC to mop it up, but this doesn't seem to be happening. I must not be understanding something deeper here, and I would appreciate some help with fixing this leak. Thanks in advance!
This is by design, your test program is supposed to exhibit runaway memory usage. You can see the underlying reason from Taskmgr.exe. Use View + Select Columns and tick "Handles". Observe how the number of handles for your process is steadily increasing. Memory usage goes up along with that, reflecting the unmanaged memory used by the handle objects.
The design choice was a very courageous one, the CLR uses 5 operating system objects per thread. Plumbing, used for synchronization. These objects are themselves disposable, the design choice was to not make the Thread class implement IDisposable. That would be quite a hardship on .NET programmers, very difficult to make the Dispose() call at the right time. Courage that wasn't exhibited in the Task class design btw, causing lots of hand-wringing and the general advice not to bother.
This is not normally a problem in a well-designed .NET program. Where the GC runs often enough to clean up those OS objects. And Thread objects are creating sparingly, using the ThreadPool for very short running threads like your test program uses.
It can be, we can't see your real program. Do beware of drawing too many conclusions from such a synthetic test. You can see GC statistics with Perfmon.exe, gives you an idea if it is running often enough. A decent .NET memory profiler is the weapon of choice. GC.Collect() is the backup weapon. For example:
static void Main(string[] args) {
int cnt = 0;
while (true) {
Thread test_thread = new Thread(() => test());
test_thread.Start();
if (++cnt % 256 == 0) GC.Collect();
Thread.Sleep(20);
}
}
And you'll see it bounce back and forth now, never getting much higher than 4 MB.
The following code is a simplified example of an issue I am seeing. This application consumes approx 4GB of memory before throwing an exception as the dictionary is too big.
class Program
{
static void Main(string[] args)
{
Program program = new Program();
while(true)
{
program.Method();
Console.ReadLine();
}
}
public void Method()
{
WasteOfMemory memory = new WasteOfMemory();
Task tast = new Task(memory.WasteMemory);
tast.Start();
}
}
public class WasteOfMemory
{
public void WasteMemory()
{
Dictionary<string, string> aMassiveList = new Dictionary<string, string>();
try
{
long i = 0;
while (true)
{
aMassiveList.Add(i.ToString(), "I am a line of text designed to waste space.... I am exceptionally useful........");
i++;
}
}
catch(Exception e)
{
Console.WriteLine("I have broken myself");
}
}
}
This is all as expected, although what we cannot currently work out is when this memory should be released from the CLR.
We have let the task complete and then simulated a memory overload situation, but the memory consumed by the dictionary is not released. As the OS is running out of memory, is it not putting pressure on the CLR to release the memory?
However and even more confusing, if we wait until the task has completed, then hit enter to run the task again the memory is released, so obviously the previous dictionary has been garbage collected (hasn't it?).
So, why is the memory not being released? And how can we get the CLR to release the memory?
Any explanations or solutions would be greatly appreciated.
EDIT: Following replies, particularly Beska's, it is obvious my description of the issue is not the the clearest, so I will try to clarify.
The code may not be the best example, sorry! It was a quick crude piece of code to try to replicate the issue.
The dictionary is used here to replicate the fact we have a large custom data object, which fills a large chunk of our memory and it is not then released after the task has completed.
In the example, the dictionary fills up to the limit of the dictionary and then throws an exception, it does NOT keep filling forever! This is well before our memory is full, and it does not cause an OutOfMemoryException. Hence the result is a large object in memory, and then the task completes.
At this point we would expect the dictionary to be out of scope, as both the task and the method 'Method' have completed. Hence, we would expect the dictionary to be garbage collected and the memory reclaimed. In reality, the memory is not freed until 'Method' is called again, creating a new WasteOfMemory instance and starting a new task.
Hopefully that will clarify the issue a bit
The garbage collector only frees locations in memory that are no longer in use that are objects which have no pointer pointing to them.
(1) your program runs infinitely without termination and
(2) you never change the pointer to your dictionary, so the GC has certainly no reason to touch the dictionary.
So for me your program is doing exactly what it is supposed to do.
Okay, I've been following this...I think there are a couple issues, some of which people have touched on, but I think not answering the real question (which, admittedly, took me a while to recognize, and I'm not sure I'm answering what you want even now.)
This is all as expected, although what we cannot currently work out is when this memory should be released from the CLR.
As others have said, while the task is running, the dictionary will not be released. It's being used. It gets bigger until you run out of memory. I'm pretty sure you understand this.
We have let the task complete and then simulated a memory overload situation, but the memory consumed by the dictionary is not released. As the OS is running out of memory, is it not putting pressure on the CLR to release the memory?
Here, I think, is the real question.
If I understand you correctly, you're saying you set this up to fill up memory. And then, after it crashes (but before you hit return to start a new task) you're trying other things outside of this program, such as running other programs in Windows to try to get the GC to collect the memory, right? Hoping that the OS would talk to the GC, and start pressuring it to do it's thing.
However and even more confusing, if we wait until the task has completed, then hit enter to run the task again the memory is released, so obviously the previous dictionary has been garbage collected (hasn't it?).
I think you answered your own question...it has not been necessarily been released until you hit return to start a new task. The new task needs memory, so it goes to the GC, and the GC happily collects the memory from the previous task, which has now ended (after throwing from full memory).
So, why is the memory not being released? And how can we get the CLR to release the memory?
I don't know that you can force the GC to release memory. Generally speaking, it does it when it wants (though some hacker types might know some slick way to force its hand.) Of course, .NET decides when to run the GC, and since nothing is happening while the program is just sitting there, it may well be deciding that it doesn't need to. As to whether the OS can pressure the GC to run, it seems from your tests the answer is "no". A bit counter-intuitive perhaps.
Is that what you were trying to get at?
The memory is not being released because the scope aMassiveList is never finished. When a function returns, it releases all non-referenced resources created inside it.
In your case, aMassiveList never leaves context. If you want your function never to return you have to find a way to 'process' your info and release it instead of storing all of them forever.
If you create a function that increasingly allocates resources and never release it you will end up consuming all the memory.
GC will only release unreferenced objects, so as the dictionary is being referenced by your program it can't be released by the GC
The way you've written the WasteMemory method, it will never exit (unless the variable "i" overflows, which won't happen this year) and BECAUSE IT WILL NEVER EXIT it will keep IN USE the reference to the internal Dictionary.
Daniel White is right, you should read about how GC works.
If the references are in use, GC will not collect the referenced memory. Otherwise, how would any program work?
I don't see what you expect the CLR/GC to do here. There's nothing to garbage-collect inside one run of your WasteMemory method.
However and even more confusing, if we wait until the task has completed, then hit enter to run the task again the memory is released, so obviously the previous dictionary has been garbage collected (hasn't it?).
When you press Enter, a new task is created and started. It's not the same task, it's a new task - a new object holding a reference to a new WasteOfMemory instance.
The old task will keep running and the memory it uses will NOT be collected because the old task keeps running in background and it keeps USING that memory.
I'm not sure why - and most importantly HOW - you observe the memory of the old task being released.
Change your method to be a using statement
Example:
Using (WateOfMemory memory = new WateOfMemory())
{
Task tast = new Task(memory.WasteMemory);
tast.Start();
}
And add disposible WateOfMemoryClass (by the way your constructor is WasteOfMemory)
#region Dispose
private IntPtr handle;
private Component component = new Component();
private bool disposed = false;
public WateOfMemory()
{
}
public WateOfMemory(IntPtr handle)
{
this.handle = handle;
}
public void Dispose()
{
Dispose(true);
GC.SuppressFinalize(this);
}
private void Dispose(bool disposing)
{
if(!this.disposed)
{
if(disposing)
{
component.Dispose();
}
CloseHandle(handle);
handle = IntPtr.Zero;
}
disposed = true;
}
[System.Runtime.InteropServices.DllImport("Kernel32")]
private extern static Boolean CloseHandle(IntPtr handle);
~WateOfMemory()
{
Dispose(false);
}
#endregion
I've a class, say "MyComputation" which does a lot of computation in one long constructor. It takes typically about 20ms to run when executed on its own (with no disk i/o or network operations). 100 or so instances of this class are created by a parent class, say "ComputeParent", which queues them up in a ThreadPool as work items:
ThreadPool.QueueUserWorkItem(myComputationCall, my_computation_data);
"myComputationCall" looks like this:
public static void myComputationCall(Object my_computation_data)
{
try
{
MyDataObject data = (MyDataObject)my_computation_data;
var computation_run = new MyComputation(data.parameter1, data.parameter2);
data.result = computation_run.result;
}
finally
{
if (Interlocked.Decrement(ref num_work_items_remaining) == 0)
done_event.Set();
}
}
done_event is a static ManualResetEvent:
private static ManualResetEvent done_event;
...
done_event = new ManualResetEvent(false);
I run ComputeParent about 500 or so times, for various input parameters. So I have a lot of nested classes. The problem is that the time it takes to execute ComputeParent gradually increases. There will be a certain amount of variation between how long it takes to run each particular ComputeParent, but the amount of time increases quite steadily (geometrically, each successive iteration take longer by a longer amount).
The memory consumption of the program does not noticably increase over time though it is quite high (~300MB). Its running on a computer with 8 logical cores, and the processor use seems to be very bursty. I'm not sure what else might be relevant to the problem.
I'd prefer not to have to run ComputeParent through batch files, though the issue does not appear to arise when this is done.
If number of available threads in the ThreadPool becomes 0, and you continue to add new work items then newly added work items will "wait". This means that your ComputeParent will wait for its instances of "myComputationCall". Starting more and more ComputeParent will cause that average execution time of them will go up.
This question has been answered. Thanks to all of the posters.
For others with a similar issue, I would suggest the Task Parallel Library as suggested by Henk.